Barbara McClintock (June 16, 1902 - September 2, 1992) was an American scientist and one of the world's most distinguished cytogeneticists. She did groundbreaking research on the phenomenon of how genes in chromosomes could "move" during the breeding of maize plants. She also showed how certain genes were responsible for turning on or off physical characteristics, such as the color of leaves or individual corn kernels. She developed theories to explain the suppression or expression of genetic information from one generation of maize plants to the next that defied the common wisdom of molecular biology prevalent during the 1950s. In the early 1960s, she traveled extensively, collected maize samples that demonstrated interesting evolutionary characteristics, and mentored junior scientists and graduate students in maize genetics. She and her colleagues spent two decades assembling data on differences in South American maize, which were published in 1981 as, The Chromosomal Constitution of Races of Maize. In 1983, at the age of 81, she received the Nobel Prize for her work on "mobile genetic elements". McClintock was the first woman to receive an unshared Nobel Prize in that category. She died in1992, at the age of 90.